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Scientists have detected gravitational waves for the second time

Scientists have detected gravitational waves for the second time


LIGO does it again

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R. Hurt/Caltech-JPL

Scientists with the LIGO collaboration claim they have once again detected gravitational waves — the ripples in space-time produced by objects moving throughout the Universe. It’s the second time these researchers have picked up gravitational wave signals, after becoming the first team in history to do so earlier this year.

"The first detection wasn’t just luck."

This second detection boosts the likelihood that LIGO is truly measuring waves and not something else. "Seeing a second loud signal like this means the first detection wasn’t just luck," said Duncan Brown, a LIGO researcher and a professor of physics at Syracuse University. These two wave signals also occurred within just a few months of each other, hinting that these detections may happen pretty frequently for LIGO moving forward.

As with the original finding, these waves came from the merger of two black holes — super dense objects that form when a star collapses and dies. During the merger, these black holes rapidly spun around each other several times a second, before joining together into a single extra-dense object. The whole process generated massive gravitational waves that rippled outward at the speed of light. Those waves billowed through space for 1.4 billion years before finally reaching Earth on December 26th (or December 25th for those in the US), when LIGO’s two observatories picked them up. The discovery was detailed in new study recently accepted for publication in the journal Physical Review Letters.


An animation of two black holes merging. (NASA)

Although both of LIGO’s detections have stemmed from black hole mergers, the two events that spawned them are different. The black holes involved in today’s finding were much smaller than the first pair; these objects were about eight and 14 times the mass of our Sun, while the first ones were around 29 and 36 solar masses. Because these holes were smaller, they didn’t produce as strong of a signal. But the smaller sizes of the holes did make the resulting signal last longer. That’s because the less massive objects didn’t "pull" on each other as strongly, so their merger took more time. This produced a signal that LIGO measured for over an entire second — much longer than the 0.5 seconds of the first signal. That longer observation time allowed researchers to observe many more rotations of the merging black holes than before.

This second detection hints that black hole mergers occur pretty frequently

This second detection also hints that black hole mergers occur pretty frequently, and LIGO will be able to pick a lot of them up. LIGO’s first detection occurred in September, and this one happened just a few months later. There's even the possibility that the collaboration measured a third signal at the end of last year, but the researchers aren’t sure yet if it came from gravitational waves.

One finding could be error; a second finding makes scientists more confident. Researchers now think they can start using gravitational wave signals as a way to learn more about the types of black holes distributed throughout the Universe. "With the first detection, we actually achieved detection of gravitational waves, marking the end of a very long era," said LIGO collaborator Susan Scott, a professor of quantum science at Australian National University. "With this second detection, we’ve started the era of gravitational wave astronomy."

Gravitational waves are a big part of Albert Einstein’s theory of general relativity, which revolutionized physics when it first came out in 1916. Before that, space and time were considered fixed concepts that didn’t really have any effect on one another. General relativity changed all that by combining space and time into a single concept, space-time. The idea was that objects could actually warp and bend space-time around them; the bigger the object, the bigger its space-time imprint. And when these massive objects move, they create undulating space-time ripples, or gravitational waves, kind of like creating ripples in a pond.

Detecting these ripples is a super precise science

Up until recently, gravitational waves were the last piece of Einstein’s theory that still hadn’t been proven. That’s because detecting these ripples is a super precise science. Waves produced by the Sun and planets in our Solar System are too weak to pick up from Earth, so scientists are limited to picking up the waves from the movements of super dense objects far away — like black holes and stellar remnants called neutron stars. But even the gargantuan waves from these objects still weaken considerably by the time they reach Earth, requiring extra sensitive instrumentation to pick up.

LIGO's two observatories in Louisiana and Washington. (LIGO)

That's where LIGO comes in. The collaboration, which stands for the Laser Interferometer Gravitational-Wave Observatory, has two facilities in Louisiana and Washington specially designed to measure gravitational ripples from big merging objects. Each observatory is shaped like a large L, the "arms" of which are two vacuum-sealed tubes stretching 2.5 miles long. At the end of each arm is a mirror that is kept as still as possible. That way, whenever a gravitational wave passes, one mirror appears to move farther away from the observatory while the other appears to move closer. LIGO scientists measure this movement by timing how long it takes for lasers to bounce off of each mirror. The mirrors don’t move very much though; their relative positions only change by one ten-thousandth the size of a proton.

Using this method, LIGO made its first wave detection on September 14th, 2015, right when the collaboration started looking for signals. After that, the team continued observing around the clock until January 12th. At first, the researchers didn’t expect the observation to last that long, as they wanted stop before Christmastime. "We originally planned to stop running and give people a break around the holidays," said David Shoemaker, a LIGO collaborator and a senior research scientist at the MIT Kavli Institute. "But we asked the operators and the other people at the observatories if they could stick around and continue to watch the machine."

A LIGO test mirror. (LIG)

That turned out to be the right call, when the scientists got an alert of a potential detection late Christmas night at 11:38PM ET. During LIGO’s observation, the researchers ran a series of computer programs that continuously looked for patterns in the observatories’ data. The programs rapidly compare those patterns with thousands of predetermined templates of what a gravitational wave signal could look like. If the data matches up with a template, it’s possible that a wave just passed by. "The thing that happened on the 26th of December was that one of those templates lined up very nicely with some data," said Shoemaker.

The researchers are eager to observe waves coming from different stellar remnants

Now that LIGO has measured two black hole mergers, the researchers are eager to observe waves coming from different stellar remnants like neutron stars. Those are small, extra-dense stars leftover when a much bigger star collapses. There’s also the possibility of picking up the merger of a neutron star with a black hole. "There are so many other types of sources we’re going after that we haven’t yet detected," said Scott. "Binary black hole systems are just one of them."

But moving forward, the team expects to observe many more black hole mergers very soon when the collaboration starts its next round of observations this fall. And the more they detect, the more they can learn about how big black holes are and how many there are throughout the cosmos. "If we can understand the distributions of black holes, we can start to understand the life and death of stars, which is really a question of where we came from and where the Universe is going," said Brown. "So that’s really cool that we can map out the history and evolution of the Universe by seeing these black holes colliding into each other."

An artist rendering of a neutron star flaring brightly. (NASA)